Hydrostatic transmission systems are widely used to transmit power from prime movers to individual loads. In their most basic configuration hydrostatic transmissions consist of a first fluid working machine, which is mounted to a prime mover, and a second fluid working machine, which is connected to the load. In the case of vehicle propulsion the first machine might be connected to the IC-Engine and the second machine might be coupled to the wheels or tracks. These transmissions are often used for propulsion and work function in off highway vehicles and to drive loads in industrial applications. They have also been considered for propulsion in on-highway vehicles and transmission systems in the renewable energy sector. The penetration of these markets has been limited, mainly due to the poor part load efficiency of these systems. If this fundamental limitation can be resolved this type of transmission has the fundamental advantage that an accumulator can be installed in the transmission to store energy at times when there is excess energy available, for example during deceleration. This energy can be released at a time of renewed torque demand improving efficiency and performance. In these systems the accumulators typically work on the principle that they compress a gas, which is contained in a pressure vessel and separated from the working fluid by means of a flexible membrane or piston.
Most loads will require some degree of torque, speed and rotation direction control. This can either be achieved through control of the displacement on one or all fluid working machines employed, or through control valves in the circuit.
When used in combination with an accumulator in order to store surplus energy at times when the load is driving the second fluid working machine, and this energy is returned to the second fluid working machine when required, extra, often complex, fluid working circuits are required. Furthermore, during the charge process the net volume of working fluid inside the accumulator increases, requiring the hydrostatic transmission circuit to replace this volume in other parts of the system. Often this problem is compounded by the common requirement to maintain an elevated pressure on the suction side of the fluid working machines in order to avoid inlet cavitation. If this is the case a second accumulator is typically employed on the low-pressure side in order to maintain a stable charge pressure at times fluid is stored and released from the high-pressure storage accumulator. This will increase system cost, weight and space requirement. An alternative is to provide a charge pump capable of providing the full flow required and intaking directly from the reservoir.
Systems proposed in the past can be broadly divided into two groups, the so-called open circuit transmissions and closed circuit transmissions.
Open circuit systems typically employ one fluid line to supply the high-pressure fluid to the load and a second one for the low-pressure return reservoir. The first fluid working machine always intakes from the reservoir and pumps fluid in the direction of the high-pressure fluid line. A direction control valve in front of the second machine is typically employed in order to change the rotational direction of the second fluid working machine. The accumulator is typically connected to the high-pressure fluid line through an on/off valve. The low-pressure side is always kept at reservoir pressure. These systems typically require large diameter fluid lines on the low-pressure side in order to avoid inlet cavitation. There is further the requirement of a large reservoir in order to provide enough fluid to avoid air entering the system when the accumulator is fully charged.
Closed circuit systems are typically arranged in a symmetrical layout with two fluid lines between the first and second fluid working machine. There is no defined high or low-pressure side of the system. In order to control direction and speed of the second fluid working machine the first machine is arranged so that it can intake from either one of the two fluid lines pumping fluid into the other one, thereby controlling the rotational direction of the load. Since the high and low-pressure side of the system are changing it is not possible to passively source fluid from a reservoir in order to make up for leakage or to compensate for thermal expansion. These systems typically employ a charge pump, which feeds fluid from a reservoir into the current low-pressure side system. A so-called shuttle valve is typically employed in order to determine the current low-pressure side of the system. These systems have the advantage of a charge pressure on the current machine inlet sides, resulting in a reduction of diameter in the required fluid lines and smaller suction manifolds. However, due to the change of high and low-pressure fluid line during operation, complex valve arrangements are required in order to connect an energy storage accumulator into the system. There is further the requirement to switch a sufficient compliance to the current low-pressure side of the system in order to maintain the required inlet charge pressure during the charge and discharge of the high-pressure storage accumulator.
In both cases above and in general it is possible to increase the maximum pumping speed of a given hydraulic machine by increasing the pressure on the suction manifold. This technique can reduce the size, cost and weight of machine. The requirement of elevated inlet pressure on the suction side of the machine is only present at times of high rotational speeds, and even then only when the machine is operating in the pumping mode. It is further important to realise that the required level of charge pressure does not follow a linear relationship to the operational speed. This is due to the quadratic relationship between flow rate (thus operational speed) and pressure drop. A machine which can operate at speeds of n with an atmospheric charge pressure of 1 bar might only achieve an operational speed of 1.5*n at 4 bar charge. The upper limit of the charge pressure sets the maximum operating speed for pumping, and is limited by the shaft seals and maximum allowable tank pressures of the hydraulic machines, but the limit without charge pressure may be only a little below this maximum.